GB2484889A

GB2484889A - Ring Cam working surface compression.

Info

Publication number: GB2484889A
Application number: GB1013775.0A
Authority: GB
Inventors: Robert George Fox; William Hugh Salvin Rampen; Uwe Bernhard Pascal Stein
Original assignee: Artemis Intelligent Power Ltd
Current assignee: Artemis Intelligent Power Ltd
Priority date: 2010-08-17
Filing date: 2010-08-17
Publication date: 2012-05-02
Anticipated expiration: 2030-08-17
Also published as: JP5412580B2; KR101454959B1; GB2484889B; WO2012022953A1; EP2440778A1; JP2012526947A; CN102753817A; AU2011211437A1; GB201013775D0; KR20120058447A; US20120260765A1

Abstract

A fluid-working machine ring cam comprises plural segments having piston facing surfaces defining a working surface. The segments comprise leading and trailing end cooperating formations each having a working surface potion, which each interlock and rollers are thereby handed over smoothly from one segment to the next irrespective of alignment tolerance or wear variations. The segment piston facing surfaces are placed in compressive stress 112 by providing an unmounted segment 100 with greater curvature 102 than its support 104 and bolting down to remove a formed gap 106 and thus partially or fully compensate for tensile stress arising from the action of rollers. The segments form a wavelike cam surface describing a portion, x, of the wave. The underlying segment has a curvature, y, as a fraction of 360 degrees and x is not an integral multiple of y. Cross apertures 114 provide greater compressibility, assisting in providing compression. Segment attachment is provided through the working surface on whichever of the leading or trailing surfaces thereof is subject to lowest forces from pistons in use.

Description

1 Ring Cam and Fluid-Working Machine Including Ring Cam

3 Field of the invention

The invention relates to a ring cam for a fluid-working machine. The invention is of 6 especial relevance in applications where ring cams are subjected to particularly large 7 forces in use, and in particular to ring cams for large fluid-working machines, for 8 example for use in the nacelle of a wind turbine.

Background to the Invention

12 Fluid-working machines include fluid-driven and/or fluid-driving machines, such as 13 pumps, motors, and machines which can function as either a pump or as a motor in 14 different operating modes.

16 When a fluid-working machine operates as a pump, a low pressure manifold typically 17 acts as a net source of a working fluid and a high pressure manifold typically acts as 18 a net sink for a working fluid. When a fluid-working machine operates as a motor, a 19 high pressure manifold typically acts as a net source of a working fluid and a low pressure manifold typically acts as a net sink for a working fluid. Within this 21 description and the appended claims, the terms "high pressure" and "low pressure" 22 are relative, and depend on the particular application. In some embodiments, low 23 pressure working fluid may be at a pressure higher than atmospheric pressure, and 24 may be several times atmospheric pressure. However, in all cases, low pressure 1 working fluid will be at a lower pressure than high pressure working fluid. A fluid- 2 working machine may have more than one low pressure manifold and more than one 3 high pressure manifold.

Large displacement ring cam fluid-working machines (i.e. fluid-working machines 6 having a large rotating annular cam driving a plurality of radial pistons arranged 7 around the cam, with each piston typically reciprocating multiple times per cam 8 revolution) are known and are proposed for use in renewable energy applications in 9 which there is a low speed rotating input but a relatively high speed electrical generator (Rampen, Taylor & Riddoch, Gearless transmissions for wind turbines, 11 DEWEK, Bremen, Dec. 2006). Ring cam fluid-working machines typically have a 12 plurality of rollers rolling on a wave shaped cam and operatively connected to pistons.

13 Each piston is slideably engaged in a cylinder, the cylinder and piston together 14 defining a working chamber containing working fluid, in communication via one or more valves with high and low pressure manifolds. The pistons are each operable to 16 undergo reciprocating motion within the cylinder so as to vary the working chamber 17 volume, when the ring cam rotates, such that a cycle of working chamber volume is 18 executed, and during which working fluid may be displaced.

Ring cam fluid-working machines may be configured so that the pistons and cylinders 21 are located inside the ring cam, the ring cam having an inward facing working 22 surface, or may be configured so that the ring cam has an outward facing working 23 surface and is located inside the pistons and cylinders. Indeed, ring cam fluid-working 24 machines of either configuration are also known in which either the ring cam rotates, or the pistons and cylinders rotate. It is also possible for the ring cam to have both 26 inward and outward facing working surfaces where the ring cam is located between 27 inner and outer rings of pistons and cylinders. It is even possible for the pistons and 28 cylinders to be aligned roughly parallel with the axis of rotation, and for the ring cam 29 to have one or more axially facing working surfaces.

31 Ring cam pumps driving relatively small hydraulic motors have been proposed as 32 robust variable speed transmissions, for example, for use in wind turbine generators, 33 or tidal stream and gravity-fed hydroelectric generators.

Multi-cylinder fluid-working machines, including ring cam fluid-working machines, 36 may be variable displacement fluid-working machines (either pumps or motors, or 37 machines operable as either pumps or motors), wherein each working chamber is 1 selectable to execute an active (or part-active) cycle of working chamber volume in 2 which there is a net displacement of working fluid, or an idle cycle in which there is 3 substantially no net displacement of working fluid, by the working chamber during a 4 cycle of working chamber volume, for regulating the time-averaged net displacement of fluid from the low pressure manifold to the high pressure manifold or vice versa.

7 Large ring cam machines are difficult and expensive to repair, requiring disassembly 8 of the whole body to repair even one working chamber. This may be particularly 9 expensive in renewable energy applications because a heavy fluid-working machine must be moved from an inaccessible location (for example the nacelle of a wind 11 turbine) requiring concomitantly large and costly transportation equipment (e.g. a 12 crane). It is therefore desirable for such large scale fluid-working machines to be 13 repairable in situ, so as to reduce or obviate the requirement for transportation of 14 large components.

16 Furthermore, large fluid-working machines (such as those suitable for renewable 17 energy generation) are typically subject to particularly high internal forces and 18 pressures. For example, the pressure of the high (and indeed low pressure) working 19 fluid of a large scale ring cam fluid-working machine, of a size suitable for a wind turbine, is particularly high. Consequently the forces received by the ring cam from 21 the rollers are also high, and it is known for the ring cam working surfaces to degrade.

22 It has been proposed to assemble large scale ring cams from a number of segments, 23 and it is known for excessive wear to occur to the roller and to the working surface 24 due to discontinuities which appear on the working surface under pressure of a roller at the interface between segments. Additionally, the weight of the rotating parts 26 themselves may lead to excessive ring cam wear.

28 Accordingly, there remains a need for a fluid-working machine and a ring cam of 29 modular construction for a radial fluid-working machine, of minimum weight and having extended working lifetime.

32 Summary of the invention

34 The terms "leading" or "trailing" edge (or end or other feature) of a part of a ring cam, or segment thereof, are expressed herein in relation to the direction of rotation of the 36 ring cam in relation to the pistons, typically due to rotation of the ring cam but in some 37 embodiments due to rotation of a housing on which the pistons are mounted. In some 1 embodiments, the relative rotation of the ring cam and the pistons may be in either 2 sense (for example the sense of rotation of a given fluid working machine may be 3 reversed at certain times during operation or maintenance) and the leading and 4 trailing edges or other features are defined in relation to one of the senses of rotation.

Reference to reciprocating motion of at least one piston being coupled to rotation of 6 the ring cam relative to the at least one piston include the possibilities that either the 7 ring cam rotates, or the at least one piston rotates, or both rotate but at unequal rates.

8 In all cases rotation is about an axis through the centre of the ring cam.

According to a first aspect of the invention there is provided a ring cam for a fluid- 11 working machine having at least one piston, the ring cam comprising at least two 12 segments; each said segment having a leading cooperating formation at a leading 13 region, and a trailing cooperating formation at a trailing region; each leading 14 cooperating formation in cooperative engagement with a said trailing cooperating formation at an interlocking region; each said segment having a piston facing surface, 16 the piston facing surfaces together defining a (typically, multi-lobe) cam working 17 surface for operative engagement with the at least one piston by way of a cam 18 engaging element (such as a part of the piston, e.g. a piston shoe, or more typically a 19 roller) so as to couple reciprocating motion of the at least one piston to rotation of the ring cam relative to the at least one piston; each said leading and trailing cooperating 21 formation having a portion of the piston facing surface; 22 characterised in that each leading cooperating formation has a piston facing surface 23 which forms part of the working surface at a trailing region of the leading cooperating 24 formation and is recessed from the working surface at the leading region of the leading cooperating formation, across the interlocking region, and each trailing 26 cooperating formation has a piston facing surface which forms part of the working 27 surface at a leading region of the trailing cooperating formation and is recessed from 28 the working surface at the trailing region of the trailing cooperating formation, across 29 the interlocking region.

31 The invention also extends in a second aspect to a piston fluid-working machine 32 comprising a ring cam according to the first aspect of the action, and at least one 33 piston (typically a plurality of pistons) operatively engaging with the ring cam working 34 surface such that reciprocating motion of the at least one piston is coupled to rotation of the ring cam relative to the at least one piston.

1 The invention also extends to a method of operating a fluid-working machine having a 2 ring cam according to the first aspect, and at least one piston coupled to the ring cam 3 by way of a cam engaging element, the method comprising causing relative rotation 4 of the ring cam and the at least one piston such that the at least one cam engaging element passes smoothly from the leading cooperating formation of a first segment to 6 the trailing cooperating formation of a second segment.

8 Typically, the or each piston operatively engages with the ring cam by way of a cam 9 engaging element, which is typically a roller.

11 In known segmented ring cams, the working surface of one ring cam segment is 12 intended to be aligned with and contact the working surface of an adjacent ring cam 13 segment, such that the rollers, or other cam engaging elements, move smoothly 14 between the ring cam segments. However, in practice, there will often be significant mismatches between adjacent working surfaces. Mismatches may be present on 16 installation, or develop through wear, or be created transiently consequent to forces 17 within the fluid working machine in use (for example resulting from the rollers, or 18 other cam engaging elements, applying force to the cams). Thus, there will be 19 discontinuities leading not only to noise, but to wear on the rollers, or other cam engaging elements, or on the edges of the working surfaces of the ring cam 21 segments.

23 However, in the ring cam of the present invention, adjacent segments have 24 interlocking cooperating formations, each of which has a piston facing surface which forms part of the working surface at one end, and is recessed from the working 26 surface at the other end. There will be a location within the interlocking zone where a 27 cam engaging element would contact both cooperating formations simultaneously.

28 Thus, a roller (in rolling engagement with the ring cam), or other cam engaging 29 element, will be handed over smoothly from one segment to the next segment, by contacting the working surface of the leading cooperating formation of the first 31 segment, and then simultaneously contacting the working surface of the leading 32 cooperating formation of the first segment and the trailing cooperating formation of 33 the second segment, and then contacting only the working surface of the second 34 segment. If there are any small mismatches in the alignment of the adjacent segments, this process will happen a little sooner or earlier. Furthermore, the exact 36 position of this location may depend on manufacturing tolerances and wear.

37 However, there should not be a discontinuity such as would be found with slightly 1 mismatched parallel working surfaces. Thus, smooth handover, minimising wear, 2 can be accomplished despite manufacturing tolerances and wear in use. The 3 cooperating formations are typically recessed from the working surface gradually 4 along their length.

6 Typically the piston facing surface at the leading region of the leading cooperating 7 formation, and the piston facing surface at the trailing region of the trailing 8 cooperating formation, are recessed from the working surface by at least 0.25mm, 9 0.5mm or 1 mm in at least part of the interlocking region.

11 Typically, each cooperating formation comprises a tongue. Typically, each leading 12 cooperating formation comprises a leading tongue, having a piston facing surface 13 which forms part of the working surface at a trailing edge of the leading tongue, and 14 which is recessed from the working surface at the leading region of the leading tongue, across the interlocking region. Typically also, each trailing cooperating 16 formation comprises a trailing tongue, having a piston facing surface which forms part 17 of the working surface at a leading edge of the trailing tongue, and which is recessed 18 from the working surface at the trailing region of the trailing tongue, across the 19 interlocking region. A cooperating formation may have a plurality of tongues. A cooperating formation may comprise first and second tongues defining a groove 21 therebetween.

23 By the interlocking region, we refer to a region where cooperating formations (e.g. 24 tongues) are adjacent each other, orthogonal to the direction in which rollers, or other cam engaging elements, move along the working surface in use. Thus, the cam 26 engaging elements will extend over both cooperating formations in the interlocking 27 region for a period of time as they are handed over from one segment to the next 28 adjacent segment.

Typically, a plurality of pistons are arranged either outside the ring cam (for an 31 outward facing ring cam), or inside the ring (for an inward facing ring cam), or, in 32 some embodiments both (for a ring cam having both inward facing and outward 33 facing working surfaces). Thus, the fluid-working machine is typically a radial piston 34 fluid-working machine. However, a plurality of pistons may be arranged generally parallel to the axis of rotation of the ring cam. The plurality of pistons are typically 36 radially arranged around the ring cam, and usually equally spaced.

1 Preferably, each piston is associated with a working chamber of volume which varies 2 cyclically with reciprocating movement of the piston. Preferably, each piston is 3 slidably mounted within a cylinder, such that a working chamber is defined between 4 the cylinder and piston. Typically, the fluid-working machine comprises a body and the or each cylinder may be formed in the body. For example, the body may comprise 6 or consist of a cylinder block. In some embodiments, the or each cylinder, or the or 7 each piston, may be articulated (typically via a spherical bearing). The or each piston 8 may be restrained within the body.

The volume of the working chamber varies cyclically with rotation of the ring cam.

11 The fluid-working machine comprises a low pressure manifold and a high-pressure 12 manifold, and a plurality of valves for regulating the flow of fluid between each 13 working chamber and the low pressure and high-pressure manifold. Typically, at least 14 one said valve associated with each working chamber is an electronically controlled valve. The fluid-working machine comprises a controller operable to control one or 16 more said electronically controlled valves, on each cycle of working chamber volume 17 and in phased relation to cycles working chamber volume, to select the net volume of 18 working fluid displaced by each working chamber on each volume cycle.

Typically, each roller, or other cam engaging element, is biased against the ring cam 21 working surface. For example, each roller, or other cam engaging element, may be 22 biased against the working surface by an elastic member, such as a spring. Typically 23 the elastic member biases each piston against each roller, or other cam engaging 24 element, thereby biasing said roller (or other cam engaging element) against the working surface. Alternatively, or in addition, each roller (or other cam engaging 26 element) and/or each piston, is biased against the working surface by fluid pressure 27 from within the respective working chamber, throughout a part or all of a cycle of 28 working chamber volume. Typically, fluid from within the respective working chamber 29 is also in direct communication with each roller, or other cam engaging element, thereby to bias said roller, or other cam engaging element, against the working 31 surface, and further to separate the roller, or other cam engaging element, from the 32 piston. For example, each said piston may define a passageway extending from the 33 working chamber and into fluid communication with the roller and the adjacent 34 surface of the piston, so that high pressure fluid pools between the piston and the roller, and functions as a self-balancing fluid bearing.

1 In practice, the force exerted on each roller, or other cam engaging element, can be 2 substantial. This force varies periodically during cycles of working chamber volume 3 (and in some embodiments depends on the volume of fluid to be displaced by the 4 working chamber on a particular cycle of working chamber volume selected by the controller). In order to reduce wear, it may be that the machine is configured, or 6 operable, such that the rollers, or other cam engaging elements, bear on the 7 interlocking regions when the respective working chamber is in direct fluid 8 communication with the low pressure manifold and/or isolated from the high pressure 9 manifold.

11 The fluid-working machine may be configured (or operable) such that the rollers, or 12 other cam engaging elements, do not bear on the interlocking regions when the 13 respective working chamber is contracting (for example in embodiments where the 14 fluid-working machine is a pump). The fluid working machine may be configured (or operable) such that the rollers, or other cam engaging elements, do not bear on the 16 interlocking regions when the respective working chamber is expanding (for example, 17 in embodiments where the fluid-working machine is a motor). The fluid-working 18 machine may be configured such that, when rotation is in a first direction, the rollers, 19 or other cam engaging elements, do not bear on the interlocking regions when the respective working chamber is contracting, and when rotation is in a second direction, 21 the rollers, or other cam engaging elements, do not bear on the interlocking regions 22 when the respective working chamber is expanding (for example, in embodiments 23 where the fluid-working machine is a pump/motor operable as a pump in a first 24 direction of rotation and as a motor in a second direction of rotation).

26 In some embodiments the roller, or other cam engaging element, of a working 27 chamber does not bear on an interlocking region every cycle(for example, the roller 28 or other cam engaging element may bear on an interlocking region during only every 29 second or only every third cycle, or only every more than two, or three or more cycles) of working chamber volume.

32 Accordingly, in embodiments wherein the fluid-working machine is operable as both a 33 pump and a motor in a first direction of rotation, the fluid-working machine may be 34 configured, or operable, such that in the first direction of rotation, each cam engaging element does not bear on an interlocking region when the respective working 36 chamber is contracting, each said working chamber operable to execute a pumping 37 cycle during every cycle of working chamber volume, and each said working chamber 1 operable to execute a motoring cycle during cycles of working chamber volume in 2 which the cam engaging element does not bear on an interlocking region (every 3 second cycle, or every third cycle, or only every more than two, or three or more 4 cycles, of working chamber volume, as the case may be.

6 In some embodiments, the fluid-working machine is operable as both a pump and a 7 motor in a first direction of rotation and each cam engaging element does not bear on 8 an interlocking region only when the respective working chamber is expanding, each 9 said working chamber operable to execute a motoring cycle during every cycle of working chamber volume, and each said working chamber operable to execute a 11 pumping cycle during cycles of working chamber volume in which the cam engaging 12 element does not bear on an interlocking region.

14 The fluid-working machine may be operable as a motor in a first direction of rotation, and as a pump in a second direction of rotation, in the first direction of rotation each 16 cam engaging element does not bear on an interlocking region when the respective 17 working chamber is expanding, and in the second direction of rotation, each cam 18 engaging element does not bear on an interlocking region when the respective 19 working chamber is contracting.

21 Thus, when a roller or other cam engaging element bears on an interlocking region, 22 the fluid pressure in the working chamber is limited, in comparison to the fluid 23 pressure in the working chamber when the cam engaging element bears upon 24 another region of the working surface (i.e. another region of the working surface not comprising an interlocking region or other discontinuity).

27 The fluid-working machine may be configured (or operable) such that the rollers, or 28 other cam engaging elements, bear on the interlocking regions only when the 29 respective working chamber is expanding (for example in embodiments where the fluid-working machine is a pump). The fluid working machine may be configured such 31 that the rollers, or other cam engaging elements, bear on the interlocking regions only 32 when the respective working chamber is contracting (for example, in embodiments 33 where the fluid-working machine is a motor).

Accordingly, in embodiments wherein the fluid-working machine is operable as both a 36 pump and a motor in a first direction of rotation, the fluid-working machine may be 37 configured, or operable, such that in the first direction of rotation, each cam engaging 1 element bears on an interlocking region only when the respective working chamber is 2 expanding, each said working chamber operable to execute a pumping cycle during 3 every cycle of working chamber volume, and each said working chamber operable to 4 execute a motoring cycle during cycles of working chamber volume in which the cam engaging element does not bear on an interlocking region (every second cycle, or 6 every third cycle, or only every more than two, or three or more cycles, of working 7 chamber volume, as the case may be.

9 In some embodiments, the fluid-working machine is operable as both a pump and a motor in a first direction of rotation and each cam engaging element bears on an 11 interlocking region only when the respective working chamber is contracting, each 12 said working chamber operable to execute a motoring cycle during every cycle of 13 working chamber volume, and each said working chamber operable to execute a 14 pumping cycle during cycles of working chamber volume in which the cam engaging element does not bear on an interlocking region.

17 The fluid-working machine may be operable as a motor in a first direction of rotation, 18 and as a pump in a second direction of rotation, in the first direction of rotation each 19 cam engaging element bearing on an interlocking region only when the respective working chamber is contracting, and in the second direction of rotation, each cam 21 engaging element bearing on an interlocking region only when the respective working 22 chamber is expanding.

24 Thus, when a roller or other cam engaging element bears on an interlocking region, the fluid pressure in the working chamber is limited, in comparison to the fluid 26 pressure in the working chamber when the cam engaging element bears upon 27 another region of the working surface (i.e. another region of the working surface not 28 comprising an interlocking region or other discontinuity).

Fluid-working machines may be operable to function as a pump or a motor (in one or 31 both directions of rotation). Fluid-working machines (for example wind turbines) may 32 function for the substantial majority of time as a pump, but also be operable as a 33 motor, so as to enable the turbine blades (or other rotating apparatus) to be driven to 34 a desired orientation during maintenance. It may be advantageous in some applications for a fluid-working machine to be operable as both a pump and a motor 36 in a given direction of rotation. For example, fluid-working machines (such as wind 37 turbines) which function as a pump for the majority of the time (and the or each cam 1 engaging element does not bear on an interlocking region when the working chamber 2 is contracting) may thus advantageously be operable on only some cycles of working 3 chamber volume (i.e. when the or each cam engaging element does not bear on an 4 interlocking region) as a motor so as to position the machine. For example, fluid-working machines (such as wind turbines) which function as a pump for the majority 6 of the time (and the or each cam engaging element does not bear on an interlocking 7 region when the working chamber is contracting) may thus advantageously be 8 operable for a minority of the time as a motor (i.e. when the or each cam engaging 9 element bears on an interlocking region when the working chamber is expanding) so as to position the machine.

12 In some embodiments (for example wherein each said working chamber is selectable 13 on a cycle by cycle basis to execute an active cycle or an idle cycle and/or selectable 14 to execute a pumping cycle or a motoring cycle, or wherein the fluid-working machine is configured to conduct a sequence of active cycles and idle cycles, or pumping 16 cycles and motoring cycles in respective first and second directions of rotation) the 17 fluid-working machine is operable (or configured), to limit the working fluid pressure in 18 the said working chamber (in comparison to the fluid pressure in the working 19 chamber when the cam engaging element bears upon another region of the working surface) when the rollers, or other cam engaging elements, bear on an interlocking 21 region (in comparison to the fluid pressure in the working chamber when the cam 22 engaging element bears upon another region of the working surface). Preferably the 23 fluid pressure is limited to a pressure substantially lower than the maximum pressure 24 during a typical active cycle of working chamber volume. For example, the may be limited to less than 50 Bar, 100 Bar or 200 Bar. The pressure may be limited to less 26 than 50%, or less than 25%, of the maximum rated operating pressure of the working 27 chamber, or the maximum pressure during a typical active cycle of working chamber 28 volume.

Preferably the pressure is limited when the rollers, or other cam engaging elements, 31 bear on an interlocking region by the controller selecting the net volume of working 32 fluid to be displaced by the said working chamber during a cycle of working chamber 33 volume. The net volume of working fluid displaced by a working chamber during a 34 cycle of working chamber volume may be selected in advance of the respective cycle of working chamber volume.

1 It may be that the net volume of fluid to be displaced by a said working chamber 2 during a cycle of working chamber volume (i.e. an active, idle, motoring or pumping 3 cycle), is selected or selectable responsive to the pressure in the high pressure 4 manifold and/or the position of each said roller (or other cam engaging element) in relation to each said interlocking region, so as to limit the working fluid pressure in the 6 said working chamber.

8 For example, the pressure in the high pressure manifold of a fluid working machine of 9 a wind turbine may vary depending on wind speed.

11 In some embodiments, the controller is operable to control the (by opening, closing or 12 prevention of the opening or closing) one or more electronically controlled valves 13 (between a said working chamber and the high and/or low pressure manifolds) to 14 select a volume of working fluid to be displaced, or to prevent displacement of working fluid, by a said working chamber during a cycle of working chamber volume, 16 when the associated roller, or other cam engaging element, bears on an interlocking 17 region, to thereby limit the working fluid pressure in the said working chamber when 18 the roller, or other cam engaging element, bears on an interlocking region.

In some embodiments, each said working chamber is operable to execute a part 21 active cycle, in which there is a net displacement of a volume of fluid which is less 22 than the maximum volume of fluid that the working chamber is operable to displace.

23 Accordingly, the controller may be operable to control the one or more electronically 24 controlled valves to select a part active cycle of a said working chamber when the associated roller, or other cam engaging element, bears on an interlocking region, to 26 thereby limit the working fluid pressure in the said working chamber during that 27 portion of said cycle of working chamber volume when the roller, or other cam 28 engaging element, bears on an interlocking region.

The working surface may comprise further discontinuities and the fluid-working 31 machine may be operable to limit the working-fluid pressure in the said working 32 chamber when the rollers, or other cam engaging elements, bear on a discontinuity.

34 Accordingly, the method may comprise selecting, on a cycle by cycle basis, one or more of an active pumping cycle and active motoring cycle or an idle cycle, of one or 36 more of the said working chambers, so as to limit the pressure in one or more of 37 working chambers, when a cam engaging element bears on an interlocking region (or 1 other discontinuity in the working surface). The method may comprise causing the 2 controller to select an active pumping cycle and active motoring cycle or an idle cycle, 3 of one or more of the said working chambers.

It may be that the axis of movement of each piston is coplanar with the ring cam, but 6 does not extend directly radially from a centre axis of the ring cam. Instead, the axis 7 of movement of each piston is preferably canted, i.e. does not extend directly away 8 from the centre axis of the ring cam. This reduces shearing forces acting on the 9 pistons slidably mounted within cylinders.

11 Typically, the working surface of the ring cam is wavelike (defining at least one and 12 typically a plurality of waves). The waves may be sinusoidal, although there is 13 typically some departure from sinusoidal form. Some or all segments may have a 14 piston facing surface defining a portion of wave. In some embodiments, one or more of the or each said segment comprises a piston facing surface defining more than 16 one wave, or a plurality of waves. Thus, the roller, or other cam engaging element, of 17 a working chamber does not bear on an interlocking region every cycle of working 18 chamber volume and may bear on an interlocking region only every more than one 19 cycle of working chamber volume (which may be an integer or non-integer number cycles of working chamber volume). The roller, or other cam engaging element, of a 21 working chamber may bear on an interlocking region only every two (or more than 22 two) cycles of working chamber volume. Thus some or all segments may comprise 23 more than one crest of the wavelike surface. Some or all segments may comprise 24 more than one trough of the wavelike surface. The segments forming a ring cam may all be the same as each other, or there may be two or more shapes of segment 26 forming the ring cam.

28 The ring cam may be mounted on a rotatable shaft. The rotatable shaft may be 29 hollow. It may be that the ring cam rotates and the at least one piston remain in place. It may be that the ring cam is stationary and the least one piston rotates 31 relative to the ring cam. It may be that, both the ring cam and the at least one piston 32 may rotate, but with different rates or directions of rotation, such that there is relative 33 rotation between the ring cam and the at least one piston.

Preferably, the most leading tip of the leading cooperating formation of a segment, or 36 the most trailing region of the trailing cooperating formation of a segment, are 37 smooth. By avoiding sharp edges, wear can be reduced.

2 Preferably, some or all segments comprise a slip-resisting formation, to resist slip of 3 the segment relative to a cam segment support. For example, one or more segments 4 may comprise a spline or groove to fit into a cooperating groove or spline of a cam segment support, or a groove to receive a keying member which also fits into a 6 groove on a cam segment support. Preferably the cam segment support comprises 7 the rotatable shaft.

9 Preferably, the piston facing surfaces of the cooperating formations of adjacent segments which engage in an interlocking region, cross at an angle of less than 11 180.00 (but typically greater than 170.00).

13 Thus, the invention extends in a third aspect to a ring cam for a fluid-working machine 14 having at least one piston, the ring cam comprising at least two segments; each said segment having a leading cooperating formation at a leading region, and a trailing 16 cooperating formation at a trailing region; each leading cooperating formation in 17 cooperative engagement with a said trailing cooperating formation at an interlocking 18 region; each said segment having a piston facing surface, the piston facing surfaces 19 together defining a (typically, multi-lobe) cam working surface for operative engagement with the at least one piston (typically by way of a cam engaging element, 21 such as a roller) so as to couple reciprocating motion of the at least one piston to 22 rotation of the ring cam or the at least one piston relative to the other; characterised in 23 that the piston facing surfaces of the cooperating formations of adjacent segments 24 which engage in an interlocking region cross at an angle of less than 180.0° (but typically greater than 170.0°).

27 Because the piston facing surfaces of cooperating formations of adjacent segments 28 which engage in an interlocking region cross at an angle of less than 180.0°, a roller, 29 mounted to a piston, rolling from one segment to an adjacent segment, will briefly have a point of contact with the working surface of each of the two adjacent 31 segments, thereby transferring force gradually from one segment to the next 32 segment. Even if there is a slight mismatch between the piston facing surfaces of 33 adjacent pistons, a roller may still pass over the resulting discontinuity if it is small 34 relative to the curvature of the ring cam and the said angle.

36 By crossing at an angle, we refer to the angle at which a plane which is coplanar with 37 the piston facing surface of a cooperating formation at an interlocking region and a 1 plane which is coplanar with the piston facing surface of an adjacent cooperating 2 formation intersect.

4 The invention also extends in a fourth aspect to a fluid-working machine comprising a ring cam according to the third aspect of the invention, and at least one piston 6 (typically a plurality of pistons), the at least one piston coupled to a roller, the at least 7 one roller in rolling engagement with the ring cam working surface such that 8 reciprocating motion of the at least one piston is coupled to rotation of the ring cam 9 relative to the at least one piston.

11 The invention extends in a fifth aspect to a ring cam for a fluid-working machine 12 having at least one piston, the ring cam comprising at least two segments; said 13 segments having a piston facing surface, the piston facing surfaces together defining 14 a (typically multi-lobe) cam working surface for operative engagement with the at least one piston by way of a cam engaging element (such as a part of the piston, e.g. 16 a piston shoe, or more typically a roller) so as to couple reciprocating motion of the at 17 least one piston to rotation of the ring cam relative to the at least one piston 18 characterised in that the piston facing surface of each segment is held in 19 compression.

21 Thus, when forces from the pistons (coupled through cam engaging elements such 22 as rollers or piston shoes) bear on the piston facing surface of each segment, the 23 resulting tensile stress is partially or fully cancelled out by the compression of the 24 piston facing surface of the segment, reducing or avoiding tensile stresses which could otherwise reduce the working life of the segment. The segment is typically 26 made from metal, such as steel, which is stronger in compression than in tension.

28 Preferably the piston facing surface is held in tangential (also known as hoop) 29 compression. By tangential compression is meant that the piston facing surface of each segment is compressed in the direction tangential to (and around) the piston 31 facing surface. Preferably the compression of the piston facing surface is greater than 32 50 MPa, 100 MPa or 200 MPa, in the direction tangential to the piston facing surface.

34 In practice, as well as the piston facing surface of each segment, at least a region adjacent the surface of each segment will be held in compression. Typically, a 36 compressed region extends into the segment from the piston facing surface, wherein 37 the tangential compression is preferably greater than 50 MPa, 100 MPa or 200 MPa.

1 For example, tangentially compressive forces may be present in the segment to a 2 depth of greater than 1 mm, 2 mm or 5 mm from the piston facing surface.

4 By being held in compression, we refer to there being compressive strain in the absence of other forces, such as forces from a piston. Thus, the segments are 6 elastically deformed. Typically, the or each segment would adopt a different shape 7 were it not held in compression. Thus, the segment is typically retained such that at 8 least the piston facing surface is held in compression, and typically tangential 9 compression, by one or more fixtures. The one or more fixtures may be releasable, to enable the segments to be removed. For example, the segments may be individually 11 removable to enable them to be tested, maintained or replaced.

13 Typically each said segment has an inherent curvature which the segment would 14 adopt without external forces and each said segment is held with a different curvature, thereby holding the piston facing surface of each segment in compression, 16 and typically tangential compression. By the inherent curvature we refer to the 17 curvature which the segment would adopt if there were no external forces acting on it, 18 such as forces arising from pistons or from the segment being held under elastic 19 deformation by one or more fixtures.

21 Typically, the ring cam comprises a cam segment support, such as a drum, and each 22 segment is fixed to the cam segment support by one or more fixtures. Typically, each 23 segment has a support facing surface opposite the piston facing surface.

It may be that each segment comprises one or more through bores extending 26 between the support facing surface and piston facing surface for receiving one or 27 more fixtures, such as bolts, to retain the segment on the cam segment support, with 28 the piston facing surface under compression. Thus, the piston facing surface of each 29 segment is typically perforated by the through bores. Each segment may comprise indentations in the side of the segment, extending from the piston facing surface to 31 the one or more fixtures.

33 The ring cam may have an outward facing working surface for operative engagement 34 with pistons radially outward of the ring cam, wherein each said segment is retained with a lesser curvature than its inherent curvature. Thus, the cam segment support 36 may define a first radius of curvature and each segment may have an inherent 37 curvature with a second radius of curvature, wherein the first radius of curvature is 1 greater than the second radius of curvature. The first radius of curvature may be 2 defined by the configuration of segment retaining formations (such as bolt holes) on 3 the cam segment support (which need not extend continuously between the segment 4 retaining formations). Each segment may be retained, with the piston facing surface in compression, by one or more bolts extending through said through bores to the 6 cam segment support. Preferably the first radius of curvature is at least 0.05% or 7 0.1 % greater than the second radius of curvature and the first radius of curvature may 8 be between 0.1% and 0.5%, or in some embodiments between 0.2% and 0.3%, 9 greater than the second radius of curvature.

11 The ring cam may have an inward facing working surface for operative engagement 12 with pistons radially inward of the ring cam, wherein each said segment is retained 13 with a greater curvature than its inherent curvature. Thus, the cam segment support 14 may define a first radius of curvature and each segment may have an inherent curvature with a second radius of curvature, wherein the first radius of curvature is 16 less than the second radius of curvature. The first radius of curvature may be defined 17 by the configuration of segment retaining formations (such as bolt holes) on the cam 18 segment support which need not extend continuously between the segment retaining 19 formations. Each segment may be retained, with the piston facing surface in compression, by one or more bolts extending through said through bores (or 21 indentations) to the cam segment support. Preferably the first radius of curvature is at 22 least 0.1% or 0.5% less than the second radius of curvature and the first radius of 23 curvature may be between 0.1% and 0.5%, or in some embodiments between 0.2% 24 and 0.3%, greater than the second radius of curvature.

26 Each said segment may comprise one or more compressible zones beneath the 27 piston facing surface (closer, and typically much closer, to the piston facing surface 28 than the support engaging surface), the compressible zones comprising a medium 29 having greater compressibility than the surrounding material of the segment.

31 The zones may extend partially or entirely across the segment (for example, 32 substantially parallel to the axis of rotation of the ring cam). The compressible zones 33 may be holes or voids in the material of said segments, for example through-bores 34 extending between opposite sides of the segment. The compressible zones may comprise any other suitable compressible medium.

37 Each said segment may comprise a plurality of compressible zones.

2 Compressible zones advantageously facilitate the generation of compression in the 3 said piston facing surfaces. The compressible zones may enable the generation of 4 greater tangential compression for a given amount of force exerted by the fixtures.

6 The invention extends in an seventh aspect to a fluid-working machine comprising a 7 ring cam according to the sixth aspect of the invention, and at least one piston 8 (typically a plurality of pistons) operatively engaging with the ring cam working 9 surface by way of a cam engaging element (such as a part of the piston or e.g. a roller) such that reciprocating motion of the at least one piston is coupled to rotation 11 of the ring cam relative to the at least one piston.

13 In an eighth aspect, the invention extends to a method of fitting a ring cam segment 14 to form a ring cam according to the sixth aspect, the method comprising the step of mounting a ring cam segment so that the piston facing surface of the ring cam 16 segment forms part of the said working surface while concomitantly elastically 17 deforming the ring cam segment to compress the piston facing surface of the 18 segment.

The cam segment support may extend continuously between at least two said 21 fixtures and typically between each said fixture, and preferably the step of mounting a 22 ring cam segment comprises bringing the support facing surface of the segment into 23 engagement with the cam segment support at or near to the leading and trailing ends 24 of the segment, such that there is a gap between the support facing surface and the support extending at least part way between the leading and trailing ends of the 26 segment (typically across the centre of the segment) and elastically deforming the 27 segment to reduce (and preferably, eliminate) the gap. The segment may be 28 elastically deformed when bolts connecting the segment to the cam segment support 29 are tensioned.

31 According to a ninth aspect of the invention there is provided a ring cam segment 32 having a working surface portion for operative engagement with a piston by way of a 33 cam engaging element (such as a part of the piston, e.g. a piston shoe, or more 34 typically a roller), the working surface portion describing a proportion, x, of a repeating wave (which may be generally sinusoidal), the segment having a curvature, 36 the segment underlying the working surface portion curving by a fraction, y, of 360°, 37 wherein x is not an integral multiple of y.

2 Typically, the ring cam segment requires to be flexed by at least 0.010 to be fitted into 3 a ring cam (that is to say, the relative orientation of the leading and trailing ends 4 required to be changed by at least 0.010) The ring cam segment may be required to be flexed by 0.1° and in one embodiment is required to be flexed by 0.05°. Typically, 6 the ring cam segment requires to be flexed by least 0.05% or 0.1% (that is to say that 7 the radius of curvature of the segment may be changed by between 0.1% and 0.5%, 8 or in some embodiments between 0.2% and 0.3%).

Thus, each segment has a working surface portion which describes a proportion 11 (which may be greater than, or less than, or equal to unity) of a repeating wave.

12 However, the curvature of the segment, underlying the working surface portion, is not 13 such that a plurality of the segments could be fitted together to form a ring cam 14 having a continuous working surface comprising an integer number of waves without the segments being flexed and thereby elastically deformed. The segments are 16 configured so that to form a ring cam having a continuous working surface comprising 17 an integer number of waves, the segments must be flexed in the appropriate sense 18 such that the working surface portions of the segments are held in compression.

According to a tenth aspect of the invention there is provided a fluid-working machine 21 comprising a ring cam, a low pressure manifold, a high pressure manifold, at least 22 one piston defining a working chamber, and at least one valve (which may be an 23 electronically controlled valve, typically an electronically controlled face sealing 24 poppet valve) associated with the or each working chamber for connecting the working chamber alternately to the low or the high pressure manifold in phased 26 relationship to cycles of working chamber volume, the ring cam having a wave-like 27 cam working surface for operative engagement with the at least one piston by way of 28 a cam engaging element (such as a part of the piston, e.g. a piston shoe, or more 29 typically a roller) so as to couple reciprocating motion of the at least one piston to rotation of the ring cam relative to the at least one piston and to thereby define the 31 cycles of working chamber volume; the waves of the wave-like cam surface each 32 having a leading face and a trailing face; characterised by discontinuities in the 33 working surface located on (and typically only on) whichever of the leading face and 34 the trailing face the at least one piston exerts does least work during normal operation resulting from the flow of fluid into and out of the working chamber in 36 phased relationship to cycles of working chamber volume.

1 The said piston may do least work on one or other of the leading or trailing face 2 during each said cycle of working chamber volume. For example, the pressure within 3 a working chamber during each said cycle of working chamber volume typically 4 varies cyclically and is at a maximum when the respective cam following element is bearing on one of the leading and trailing faces (such that the force exerted on the 6 working surface is at a maximum and most work is done on the said faces), and is at 7 a minimum when the cam following element is bearing on the other of the leading and 8 trailing faces (such that the force exerted on the working surface is at a minimum and 9 least work is done on the said faces).

11 The said piston may do least work on one or other of the leading or trailing face 12 during the operating-lifetime of the fluid working machine, or of the ring cam. Least 13 work may be done on one or other of the leading or trailing face over any given time 14 period. For example, a fluid-working machine may have more than one operating mode and may have a first operating mode (which may be in a first direction of 16 rotation) in which more work is done on one of the leading or trailing face during each 17 said cycle of working chamber volume (or the majority of cycles) and a second 18 operating mode (which may be in a second direction of motion) in which more work is 19 done on the other of the leading or trailing face during each said cycle (or the majority of cycles) of working chamber volume, wherein the fluid-working machine functions in 21 the first mode for the majority of the time (for example during normal operation) and 22 the second mode for the minority of the time (for example, during maintenance), the 23 said discontinuities located on (and typically only on) whichever of the leading face 24 and the trailing face the at least one piston exerts does least work during the first mode of operation.

27 Typically, the ring cam comprises at least two segments extending around the 28 circumference of the ring cam; and a support structure to which the said segments 29 are attached; each said segment comprising a piston facing surface, the piston facing surfaces of the segments defining the working surface.

32 The said discontinuities in the working surface may be attachment means for 33 securing said segments to the support structure. The attachment means may, for 34 example, be one or more fixtures, such as bolts, extending through the working surface (typically, through a part of a segment piston facing surface which defines the 36 working surface of the ring cam). The attachment means may comprise apertures 37 through the said segments and/or recesses for receiving bolts.

2 The said discontinuities may be discontinuities between adjacent segments. It may 3 be that the plurality of segments each comprising leading and trailing cooperating 4 formations and the discontinuities in the working surface comprise interlocking regions where the leading and trailing cooperating formations of adjacent segments 6 overlap across the respective interlocking region.

8 Wear on the ring cam working surface and the cam engaging elements increases 9 with received force, and is greatest in regions of the working surface having a discontinuity (such as interlocking regions between ring cam segments, or 11 attachment means for securing segments to a support structure). Therefore, the fluid- 12 working machine of the invention, wherein force received in the regions of the 13 working surface of the ring cam having discontinuities over time (i.e. work done on 14 the said regions averaged over time and, in some embodiments, during each said cycle of working chamber volume) is lower in comparison to other regions. Thus, the 16 rate of wear of the working surface and cam engaging elements is reduced.

18 Typically, the said discontinuities are located only on the said whichever of the 19 leading face and the trailing face the pistons do least work during normal operation, (or do least work averaged over time, such as the operating lifetime of the fluid- 21 working machine or the ring cam) resulting from the flow of fluid into and out of the 22 working chamber in phased relationship to cycles of working chamber volume.

24 It may be that the said discontinuities are of a first type (e.g. attachment means, or said interlocking regions) and that the working surface comprises further 26 discontinuities, of a second type, which are otherwise distributed, for example, within 27 troughs between adjacent waves, or on both leading and trailing faces.

29 Typically, the fluid-working machine comprises a plurality of pistons arranged radially around the ring cam.

32 Each said cam engaging element may be biased against the working surface by fluid 33 pressure from within the respective working chamber (during a part of or, more 34 typically, all of each cycle of working chamber volume). Thus force on each said piston resulting from the pressure of working fluid in the respective working chamber 36 is transmitted to the working surface by the or each said cam engaging element and 37 bears upon the working surface (thereby doing work on the working surface). Each 1 said cam engaging element may alternatively or in addition be biased against the 2 working surface by an elastic member, such as a spring.

4 The fluid-working machine may be operable to limit the pressure in a said working chamber when a respective cam engaging element bears on a said discontinuity (to 6 thereby reduce the force exerted on the region of the working surface comprising the 7 discontinuity, such that the work done on the region of the working surface 8 comprising the discontinuity is also limited).

Preferably, the working chamber is sealed from the high pressure manifold when the 11 cam engaging element bears on the said discontinuities, in order that the pressure is 12 limited within the working chamber when the cam engaging element bears on a said 13 discontinuity. For example, the working chamber may be sealed from the high 14 pressure manifold by way of a valve (referred to herein as the high pressure valve), which is typically an electronically controlled valve (such as a face seating poppet 16 valve, which may be an electronically controlled face sealing poppet valve).

17 Alternatively, or in addition, the working chamber may be placed in fluid 18 communication with the low pressure manifold when the cam engaging element 19 bears on the said discontinuities, for example by way of a valve (such as a face seating poppet valve, which may be an electronically controlled face sealing poppet 21 valve) in order that the pressure is limited within the working chamber when the cam 22 engaging element bears on a said discontinuity.

24 Typically, a contraction stroke occurs when the cam engaging element bears upon the leading face of a wave and an expansion stroke occurs when the cam engaging 26 element bears upon the trailing face.

28 It may be that the fluid-working machine is a pump and each said discontinuity is 29 located in a trailing face. In a pump, fluid will typically be received from the low pressure manifold during an expansion stroke while the respective cam engaging 31 element bears on the trailing face and so each discontinuity will coincide with a period 32 of relatively low pressure in the working chamber.

34 It may be that the fluid-working machine is a motor and each said discontinuity is located in a leading face. In such a motor, fluid will typically be displaced to the low 36 pressure manifold during a contraction stroke while the respective cam engaging 1 element bears on the leading face and so each discontinuity will coincide with a 2 period of relative low pressure in the working chamber.

4 It may be that the said discontinuities are located in the working surface of only some of whichever of the leading face and the trailing face the pistons do least work during 6 operation resulting from the flow of fluid into and out of the working chamber in 7 phased relationship to cycles of working chamber volume in a first operating mode.

9 It may be that the fluid-working machine has a second operating mode in which it executes whichever of active pumping or active motoring cycles of working chamber 11 volume cause the pistons to do a greater amount of work on the said whichever of the 12 leading face and the trailing face the pistons do least work during operation in the first 13 operating mode, and in which said active pumping or active motoring cycles are 14 carried out selectively when the cam engaging element bears on the said whichever of the leading face and the trailing face the pistons do least work during operation in 16 the first operating mode do not have the said discontinuities.

18 The first operating mode may be pumping and the faces on which the pistons do least 19 work (or exert least force) during operation in the first operating mode may be the trailing faces and the second operating mode may be motoring.

22 The first operating mode may be motoring and the faces on which the pistons do 23 least work (or exert least force) during operation in the first operating mode may be 24 the leading faces and the second operating mode may be pumping.

26 Each said working chamber may comprise one or more electronically controllable 27 valves, and the fluid-working machine may comprise a controller, operable to control 28 the or each electronically controllable valve. Each said working chamber may be 29 selectable by the controller, on a cycle by cycle basis, to conduct an active cycle (where there is a net displacement of working fluid) or an idle cycle (where there is 31 substantially no net displacement of fluid), by virtue of control of the or each 32 electronically controllable valve. Similarly, each said working chamber may be 33 selectable to conduct an active pumping cycle or an active motoring cycle, on a cycle 34 by cycle basis. It may be that the controller executes a program stored on a computer readable storage medium in use and the program determines whether least 36 force is exerted, or least work is done, in use on the leading or trailing faces.

1 Thus, the invention extends in an eleventh aspect to a ring cam for a fluid working 2 machine, the ring cam comprising a working surface for operative engagement with 3 at least one piston by way of a cam engaging element (such as a part of the piston, 4 e.g. a piston shoe, or more typically a roller) so as to couple reciprocating motion of the or each piston to rotation of the ring cam relative to the or each piston and to 6 thereby define the cycles of working chamber volume; the working surface 7 comprising a plurality of waves having leading and trailing faces, wherein the working 8 surface comprises discontinuities either in the leading faces or the trailing faces of the 9 said plurality of waves.

11 By the leading and trailing faces we refer to the faces of each wave on which each 12 cam engaging element first and last engages, in use of the ring cam in a fluid-working 13 machine. It may be that the ring cam is intended for use in either of two orientations 14 in which case whichever sense is considered leading or trailing is arbitrary.

16 Every wave of the working surface may comprise a said discontinuity, or the working 17 surface may comprise discontinuities mutually spaced by more than one wave length 18 (for example only every one and a half waves, or only every second, or only every 19 third wave). The working surface may comprise one or more waves not having a discontinuity therein.

22 Typically, the ring cam comprises at least two segments extending around the 23 circumference of the ring cam; and a support structure to which the said segments 24 are attached; each said segment comprising a piston facing surface, the piston facing surfaces of the segments defining the working surface.

27 It may be that the said discontinuities are discontinuities within the piston facing 28 surfaces of the segments (rather than discontinuities between segments). The said 29 discontinuities in the working surface may be attachment means for securing said segments to the support structure. The attachment means may, for example, be one 31 or more fixtures, such as bolts, extending through the working surface (typically, 32 through a part of a segment piston facing surface which defines the working surface 33 of the ring cam). The attachment means may comprise apertures through the said 34 segments or recesses for receiving bolts.

36 However, it may be that the discontinuities are interfaces between adjacent 37 segments. For example, it may be that each of the plurality of segments each 1 comprising leading and trailing cooperating formations and the said discontinuities in 2 the working surface comprise interlocking regions where the leading and trailing 3 cooperating formations of adjacent segments overlap across the respective 4 interlocking region.

6 Typically, the said discontinuities are located only on the said leading faces or only on 7 the said trailing faces.

9 It may be that the said discontinuities are located only in some of the said leading or trailing faces. For example, in alternate leading or trailing faces, or every three or 11 four leading or trailing faces, or in two out of every three, or three out of every four 12 leading or trailing faces.

14 In some embodiments there is more than one discontinuity (for example one and a half, or two, or more than two) per wave (and therefore per cycle of working chamber 16 volume).

18 The waves of the working surface may be sinusoidal.

According to a twelfth aspect of the invention, of the invention there is provided a 21 method of operating a fluid-working machine, the fluid-working machine comprising a 22 ring cam, a low pressure manifold, a high pressure manifold, at least one piston 23 defining a working chamber, and at least one valve (which may be an electronically 24 controlled valve, typically an electronically controlled face sealing poppet valve) associated with the or each working chamber for connecting the working chamber 26 alternately to the low or the high pressure manifold in phased relationship to cycles of 27 working chamber volume, the ring cam having a wave-like cam working surface for 28 operative engagement with the at least one piston by way of a cam engaging element 29 (such as a part of the piston, e.g. a piston shoe, or more typically a roller) so as to couple reciprocating motion of the at least one piston to rotation of the ring cam 31 relative to the at least one piston and to thereby define the cycles of working chamber 32 volume; the waves of the wave-like cam surface each having a leading face and a 33 trailing face; the leading or trailing faces comprising discontinuities, the method 34 characterised by the at least one piston doing least work during normal operation resulting from the flow of fluid into and out of the working chamber in phased 36 relationship to cycles of working chamber volume on whichever of the leading face 37 and the trailing face comprise the discontinuities.

2 It may be that only some of the said leading or trailing faces comprise said 3 discontinuities.

According to a thirteenth aspect of the invention, there is provided a method of 6 operating a fluid-working machine, the fluid-working machine comprising a ring cam, 7 a low pressure manifold, a high pressure manifold, at least one piston defining a 8 working chamber of cyclically varying volume, and at least one electronically 9 controlled valve associated with the or each working chamber for connecting the working chamber alternately to a said low pressure manifold and a said high pressure 11 manifold in phased relationship to cycles of working chamber volume; the ring cam 12 having a working surface for operative engagement with the or each piston by way of 13 a cam engaging element (such as a part of the piston, e.g. a piston shoe, or a roller) 14 so as to couple reciprocating motion of the or each piston to rotation of the ring cam relative to the or each piston and to thereby define the cycles of working chamber 16 volume; one or more regions of the working surface having a discontinuity; 17 the method characterised by limiting the working fluid pressure in a said working 18 chamber when the respective cam engaging element (i.e. the cam engaging element 19 through which the piston defining the said working chamber engages within the cam working surface) bears on a said discontinuity in the working surface.

22 Thus, the fluid pressure in the working chamber when the cam engaging element 23 bears upon a discontinuity (and, in some embodiments, the leading face or trailing 24 face in which the discontinuity is located) is typically less than the pressure in the working chamber when the cam engaging element bears upon another region of the 26 working surface. Consequently, the work done by the said pistons on the working 27 surface, and the wear of the working surface, in the region of a discontinuity is 28 reduced.

The ring cam may be a ring cam according to the eleventh aspect of the invention.

32 It may be that the working fluid pressure in the said working chamber is limited by 33 sealing the working chamber from the high pressure manifold when the cam 34 engaging element passes over (i.e. bears on) the discontinuity, for example, by controlling the timing of opening or closing of a said electronically controlled valve 36 regulating the flow of fluid between the working chamber and the high pressure 37 manifold.

2 It may be that the ring comprises a plurality of waves having leading and trailing faces 3 and the said discontinuities are located in one of the leading or the trailing faces and 4 the working fluid pressure in the said working chamber is limited by synchronising active cycles of working chamber volume with rotation of the ring cam such that the 6 point in each active cycle of working chamber volume where working chamber 7 pressure is greatest occurs while the cam engaging element bears on the other of the 8 leading or the trailing faces.

It may be that the ring cam comprises a plurality of waves having leading and trailing 11 faces and the said discontinuities are located in only some of the said leading or only 12 some of the said trailing faces (for example on alternate leading faces, or only every 13 third leading face, or on alternate trailing faces, or only every third trailing face); 14 and the fluid-working machine has a first operating mode (e.g. pumping) in which the pressure in each working chamber exceeds a threshold when (and typically only 16 when) the respective cam engaging element bears on the other of the said leading or 17 the trailing faces (i.e. those in which the discontinuities are not located); 18 and (typically) the pressure in each working chamber does not exceed the threshold 19 when the respective cam engaging element bears on each of the said discontinuities (or when the respective cam engaging element bears on the portion comprising a 21 said discontinuity, or all of each of the said leading or trailing faces in which the 22 discontinuities are located); 23 and a second operating mode (e.g. motoring, or a second pumping mode in which the 24 ring cam rotates in the opposite direction to the first mode) in which the pressure in each working chamber does not exceed the threshold when the cam engaging 26 element bears on the said discontinuities (or on the said leading or trailing faces in 27 which the discontinuities are located).

29 It may be that working fluid pressure in the said working chamber is limited (for example in a second operating mode) by selecting the timing of active cycles of 31 working chamber volume in the said operating modes so that the pressure does not 32 exceed the threshold the majority, or preferably all, of the times when the cam 33 engaging element bears on a said discontinuity (or, in some embodiments, on the 34 said leading or trailing faces in which the discontinuities are located).

36 The threshold may be a pressure value, or may be a range of values. The threshold 37 may be selected as a proportion of the pressure in the high pressure manifold, or as a 1 proportion of the maximum rated operating pressure of the said working chambers, or 2 the threshold may be empirically determined in relation to the physical properties of 3 the ring cam. The threshold may be varied according to the operating requirements of 4 the fluid working machine.

6 Typically, the net displacement of working fluid on each cycle of working chamber 7 volume is determined by controlling the said one or more electronically controllable 8 valves. Typically, on each cycle of working volume, a decision is made as to whether 9 to carry out an active cycle in which a net displacement of working fluid is made (e.g. an active pumping cycle or an active motoring cycle) or an idle cycle in which no net 11 displacement of fluid is made.

13 Preferably the fluid pressure within a working chamber is limited to a pressure 14 substantially lower than the maximum pressure during a typical active cycle of working chamber volume when the respective cam engaging element bears on a said 16 discontinuity. Accordingly, the threshold is typically substantially lower than the 17 maximum pressure during a typical active cycle of working chamber volume. For 18 example, the pressure may be limited to less than 50 Bar, 100 Bar or 200 Bar. The 19 pressure may be limited to less than 50%, or less than 25%, of the maximum rated operating pressure of the working chamber, or the maximum pressure during a 21 typical active cycle of working chamber volume.

23 Typically, the pressure within a working chamber during a cycle of working chamber 24 volume varies cyclically and is at a maximum when the respective cam following element is bearing on one of the leading and trailing faces, and is at a minimum when 26 the cam following element is bearing on the other of the leading and trailing faces.

27 Accordingly, the working chamber typically executes an active cycle in which the 28 pressure within the working chamber reaches a maximum when the respective cam 29 engaging element bears on leading or trailing faces not having a discontinuity therein.

In some embodiments, it may be that the working chamber is operable to execute an 31 active cycle in which the pressure within the working chamber reaches a maximum 32 while the respective cam engaging element bears on a leading or trailing face having 33 a discontinuity (e.g. attachment means) therein only if a measured (or predicted) 34 pressure, such as the pressure in the high pressure manifold measured by a pressure sensor, is below the threshold.

1 It may be that the net volume of fluid to be displaced by a said working chamber 2 during a cycle of working chamber volume is selected or selectable responsive to the 3 pressure in the high pressure manifold and/or the position of each said roller (or other 4 cam engaging element) in relation to each said discontinuity (e.g. attachment means).

7 In some embodiments, the controller is operable to control (by opening, closing or 8 prevention of the opening or closing) the one or more of the electronically controlled 9 valves to select a volume of working fluid to be displaced, or to prevent displacement of working fluid, by a said working chamber during a cycle of working chamber 11 volume, when the associated roller, or other cam engaging element, bears on a 12 discontinuity, to thereby limit the working fluid pressure in the said working chamber.

14 It may be that the net volume of fluid to be displaced by a said working chamber during a cycle of working chamber volume is selected or selectable responsive to the 16 pressure in the high pressure manifold and/or the position of the or each cam 17 engaging element in relation to each said discontinuity, to thereby limit the working 18 fluid pressure in the said working chamber.

In some embodiments, the controller is operable to control (by opening, closing or 21 prevention of the opening or closing) the one or more of the electronically controlled 22 valves to select a volume working fluid to be displaced, or to prevent displacement of 23 working fluid by a said working during a cycle of working chamber volume, to thereby 24 limit the working fluid pressure in the said working chamber, when the associated cam engaging element, bears on a discontinuity. Thus, each said working chamber 26 may be selectable or selected, by the controller, on a cycle by cycle basis to execute 27 an active cycle or an idle cycle, or a part active cycle, or a pumping cycle or a 28 motoring cycle, so as to limit the pressure of working fluid in the said working 29 chamber, when the cam engaging element bears on a discontinuity in the working surface.

32 The method may comprise reading discontinuity location data from a computer 33 readable data storage medium (e.g. a memory) which stores data concerning the 34 location of each said discontinuity in relation to the relative orientation of the ring cam. The method may comprise reading ring cam orientation data (e.g. from a 36 sensor) and determining whether a cam engaging element associated with a working 37 chamber will or will not pass over a discontinuity on the leading or trailing face of a 1 wave of the ring cam surface during a particular cycle of working chamber responsive 2 to that data.

4 According to a fourteenth aspect of the invention, there is provided a fluid-working machine comprising a ring cam, a low pressure manifold, a high pressure manifold, at 6 least one piston defining a working chamber of cyclically varying volume, and at least 7 one electronically controlled valve associated with the or each working chamber for 8 connecting the working chamber alternately to a said low pressure manifold and a 9 said high pressure manifold in phased relationship to cycles of working chamber volume; the ring cam having a working surface for operative engagement with the or 11 each piston by way of a cam engaging element (such as a part of the piston, e.g. a 12 piston shoe, or a roller) so as to couple reciprocating motion of the or each piston to 13 rotation of the ring cam relative to the or each piston and to thereby define the cycles 14 of working chamber volume; one or more regions of the working surface having a discontinuity;_characterised by the machine being operable to limit the working fluid 16 pressure in a said working chamber when the respective cam engaging element 17 bears on a said discontinuity in the working surface.

19 Preferred and optional features described in relation to any of the first through fourteenth aspects of the invention correspond are preferred and optional features of 21 any of the first through fourteenth aspects of the invention.

23 The invention also extends to a kit of parts which, when assembled, forms a ring cam 24 according to the first aspect of the invention, or according to the third aspect of the invention, or according to the sixth aspect of the invention or according to the 26 eleventh aspect of the invention, or a fluid-working machine according to the second 27 aspect of the invention, or according to the fourth aspect of the invention, or 28 according to the seventh aspect of the invention, or according to the tenth aspect of 29 the invention, or according to the fourteenth aspect of the invention.

31 The invention also extends to a kit of parts comprising a cam segment support and a 32 plurality of cam segments according to the ninth aspect of the invention, or disclosed 33 in relation to any of the first through twelfth aspects, wherein the piston facing 34 surfaces of the plurality of cam segments together form a ring cam working surface having an integral number of waves when elastically deformed and fitted to the cam 36 segment support.

1 The invention also extends to a computer readable medium storing program code 2 which when executed on a fluid working machine controller, causes the controller to 3 carry out a method according to the twelfth or thirteenth aspect of the invention.

Description of the Drawings

7 An example embodiment of the present invention will now be illustrated with 8 reference to the following Figures in which: Figure 1(a) is a plan view of a portion of a working surface of a ring cam of a wind 11 turbine pump, defined by the piston facing surfaces of two cooperatively engaged 12 cam segments; 14 Figure 1(b) is an axial section along line A of a ring cam of a wind turbine pump, showing two cam segments secured to the turbine drive shaft, schematically 16 depicting the position of axial piston rollers, and a working chamber, in relation to the 17 working surface defined by the cam segments; 19 Figure 2 shows a schematic axial section of a portion of a cam support structure and a cam segment (a) in an un-stressed state prior to being secured to the support 21 structure and (b) secured to the support structure and in a pre-stressed state; and 23 Figure 3 is a cross-section through a section of a ring cam comprising a plurality of 24 segments having apertures in alternate trailing faces.

26 Detailed Description of an Example Embodiment

28 With reference to Figure 1(b), a portion of a ring cam 1 is formed from cam segments 29 5 and 7, secured by bolts 3 which extend from apertures 4 in the surface of the cam segments and fix the cam segments to a cam support structure 2. A plurality of 31 further cam segments (not shown) are securable to the cam support structure, to 32 make up the complete ring cam. The cam support structure is coupled to drive shaft 33 10 (which rotates in direction B in normal use) through which torque is received from 34 an energy source (e.g. the blades of a wind or tidal turbine).

36 Each cam segment has a piston facing surface 15, 16 which defines a portion of the 37 working surface of the ring cam. Thus, the ring cam has a working surface defined by 1 a plurality of cam segments secured around the circumference of the drive shaft. The 2 working surface is wave-like and comprises a plurality of waves having leading 3 surfaces 70 and trailing surfaces 72 (defined relative to the direction of rotation). The 4 waves may be generally sinusoidal although this is not essential. The piston facing surfaces preferably have a heat and/or chemical treatment applied during 6 manufacture to achieve the desired surface properties.

8 Notches in the segments 30 and shaft 32 mate with keys 34 that prevent the 9 segments rotating around the shaft, acting as a slip preventing feature. Cross bolt holes 36 are for holding side plates (120, shown in Figure 3) to the segments, which 11 prevent the rollers 9 from sliding off the rolling surface. Alternatively, a convex or 12 concave camber may be applied to the rollers and/or the piston facing surfaces to 13 achieve the same outcome.

Each segment 5,7 has at one end a trailing tongue formation 40 (being an example of 16 a trailing cooperating formation) and, at the other end, a groove formation 54 formed 17 between two leading tongue formations 46 (together forming an example of a leading 18 cooperating formation).

Leading segment 5 has a trailing edge 42 which interlocks with a leading edge 44 of 21 groove formation 54 of the trailing segment 7. The tongue and groove formations in 22 combination act as the interlocking region.

24 The faces of the tongue 48,50 and groove 52, 54 formations may be perpendicular to the shaft as shown, or may be at another angle to the shaft. The tongue and groove 26 formations may cooperate to fix the segments relative to each other by end faces 50, 27 54 and side faces 48, 52, which need not be parallel, together arranged so as to 28 make the fit between tongue and groove formations tight or loose as desired. Tongue 29 40 has cut away leading edges leaving a gap 49 to improve the fit between the segments and avoid buckling. A number of other suitable formations will present 31 themselves to those skilled in the art.

33 The piston facing surface of each segment extends continuously from the outer 34 surface of the leading tongue formations to the outer surface of the trailing tongue formation, The piston facing surface of the leading tongue formations of the trailing 36 segment 7 form part of the working surface at a trailing region of the leading tongue 37 formations, but is gradually recessed along the length of the leading tongue 1 formations, such that the piston facing surface 16 is recessed from the working 2 surface at the leading end of the trailing segment. The piston facing surface of the 3 trailing tongue formation of the leading segment 5 forms part of the working surface at 4 a leading region, but is gradually recessed along the length of the trailing tongue formation, such that the piston facing surface 15 is recessed from the working surface 6 at the trailing edge 50 of the leading segment. Thus, the piston facing surfaces of 7 each segment predominantly forms parts of the working surface of the ring cam, 8 however, there is also a part of the piston facing surface at each end of each 9 segment which does not form part of the working surface of the ring cam (and which is recessed from the working surface of the ring cam).

12 Because the piston facing surface of the tongues is recessed towards the leading end 13 of the leading cooperating formations of the trailing segment and the trailing end of 14 the trailing cooperating formation of the leading segment, the piston facing surfaces of the leading segment and the trailing segments subtend an angle of close to but 16 less than 180.0°, for example 178.0° at the interlocking region.

18 In order to form a fluid-working machine, pistons 11 are coupled to the ring cam by 19 way of rollers 9 which bear on the working surface and roll along the working surface in use so as to reciprocate the pistons within cylinders 13. The pistons and cylinders 21 together define working chambers 20 of cyclically varying volume and the volume 22 cycles of the working chambers are defined by the wave-like shape of the working 23 surface of the ring cam over which the rollers pass. The pistons are biased against 24 the working surface of the ring cam by springs (not shown) and/or by the pressure of working fluid within the working chambers. The cylinders and pistons are slightly 26 canted so that the central axis of each piston and cylinder does not extend directly 27 radially outwards from the centre of the ring cam. This reduces lateral forces of the 28 pistons acting against the cylinders when the working surface is heavily loaded in 29 use.

31 In use, either of the leading surfaces 70 or trailing surfaces 72 may be subject to the 32 greatest load, in use, depending on application (for example, whether the fluid- 33 working machine is functioning as a pump or as a motor). The axis of each of the 34 working chambers (as defined by the path of the pistons) is typically canted away from the radius of the ring cam and towards an axis which is perpendicular to 36 whichever of the leading or trailing surfaces are under greatest loading during normal 37 use. For example, if only the leading surfaces are heavily loaded (e.g. the machine is 1 rotated in direction B and used predominantly (or only) as a pump), the axis of the 2 working chamber may be canted slightly (typically in the region of 1°-10°) clockwise 3 (in relation to the orientation of Figure 1(b)) towards an axis perpendicular to the 4 leading surfaces 72.

6 A controller 17 is provided to read the angular position and speed of the shaft via a 7 shaft sensor 18 and to control the low pressure valves 19 and (optionally) high 8 pressure valves 21 for each cylinder according to a control algorithm. The low 9 pressure valves alternately place the working chamber in fluid communication with, and isolate the working chamber from, a low pressure manifold 23. The high pressure 11 valves alternately place the working chamber in fluid communication with, and isolate 12 the working chamber from, a high pressure manifold 25. The manifolds are 13 connected to sources or sinks of working fluid (not shown). The valves are ideally 14 face-seating poppet type valves, with the low pressure valve oriented to allow fluid into the working chamber and optionally out of the chamber (when controllably held 16 open) and the high pressure valve oriented to allow fluid out of the working chamber 17 and optionally into the working chamber (when controllably held open).

19 In use, the ring cam rotates in the direction B in relation to the working chambers, and rollers 9 roll over the most leading part 60 of the piston facing surface of the leading 21 tongue formation 44of the trailing segment 7 before the most trailing part 62 of the 22 piston facing surface of the trailing tongue formation 42 of the leading segment 5.

23 Because the most leading part 60 of the piston facing surface of the trailing segment 24 is below the piston facing surface 15 of the leading segment (which forms part of the working surface), the roller may smoothly transition from one segment to the next.

26 Even if there is a slight mismatch between leading and trailing segments (as can 27 result from deviations in dimensions within manufacturing tolerances), this will only 28 lead to a slight difference in the location at which the roller transfers from one 29 segment to the next but would not lead to jarring as the rollers would not encounter a discontinuity in the working surface.

32 The interlocking region is located so that the working chamber 20 is expanding when 33 its roller passes from one segment to the next, i.e. it is located at a trailing surface 72 34 of the working surface, and so the force exerted on the ring cam by the pistons, through the rollers as they pass over an interlocking region, is not at a maximum.

1 By controlling the opening and closing of the low (and optionally high) pressure 2 valves, the net volume of working fluid displaced by each chamber can be selected 3 for each cycle of working chamber volume to enable the overall fluid displacement to 4 be matched to a demand signal, such as a fluid volume demand signal or an output pressure signal. Suitable control algorithms are disclosed in EP 0 361 927, 6 EP 0 494 236 and EP 1 537 333, the contents of which are incorporated herein by 7 virtue of this reference.

9 Thus, the invention provides a mechanism to enable rollers, or other cam engaging elements, to move smoothly from one segment to next, minimising wear. However, 11 as a plurality of segments are provided, they can be individually checked, maintained 12 and replaced if need be.

14 Furthermore, the bolts 3 (functioning as attachment means) are located in bores extending from apertures 4 which are also located in the trailing surfaces 72 of the 16 working surface, and thus the force exerted on the ring cam by the pistons, through 17 the rollers as they pass over the apertures, is also not at a maximum.

19 The phase of the cycles of working chamber volume is defined by the waves in the working surface of the ring cam. In this example, the ring cam is part of a fluid 21 working pump and so the forces acting on the ring cam from the pistons are greatest 22 during the compression stroke of each working chamber which coincides with the 23 rollers passing along the leading surfaces 70. If the fluid working machine was a fluid 24 working motor (or operated for a substantial proportion of the time as a motor, or if the highest torque requirement (and thus the greatest forces transmitted to the 26 working surface) was when operating as a motor), the bolts would instead be 27 advantageously located in bores extending from apertures in the working surface of 28 the leading surfaces 70. This enables the attachment means to be provided in the 29 working surfaces (thereby enabling the ring cam and the fluid working machine to be more compact and lighter than known apparatus) while minimising wear on the 31 rollers, or other cam engaging elements, which might otherwise result from the 32 discontinuities caused by the apertures in the working surface.

34 It may be that the segments are manufactured by fracturing a continuous ring into smaller parts, for example by making a plurality of notches or imperfections in the 36 continuous ring and then expanding or otherwise overstressing said ring to fracture it.

1 Figure 2 (a) shows in exaggerated form how a segment 100 might be formed with a 2 fixing surface (a support facing surface) 102 having a smaller radius of curvature than 3 the outer surface 104 (functioning as a cam segment support) of the shaft 105 to 4 which it is to be fixed, causing there to be a gap 106 between the segment and the shaft where the segment is placed against the shaft.

7 Figure 2 (b) illustrates a fixing force 110 into the shaft, which is typically exerted by 8 attachment means (for example, bolts 3 of segments 5,7 shown in figures 1(a) and 9 (b)). Fixing force 110 deforms the segment 100 to close the gap 106, bringing the segment into cooperative engagement with the outer surface 104 (as shown in Figure 11 2(b)). A tangential compressive stress 112 is induced in the segment and, in 12 particular, in the piston facing surface 108, as a result of the deformation of the 13 segment by fixing force 110.

The tangential compressive stress is exerted generally parallel to the piston facing 16 surface over which rollers pass in use. This tangential compressive stress increases 17 the longevity of the rolling surface when subject to very high forces from passing 18 rollers. Highly loaded passing rollers would otherwise cause localised compression 19 of the rolling surface towards the shaft, which would otherwise cause tangential tensile stress in the piston facing surface. Thus, the compressive stress 112 offsets 21 some of the tensile stress. Indeed, in some alternative embodiments, the 22 compressive stress may exceed the expected tensile stress so that the piston facing 23 surface is not subject to tangential tensile stresses during normal use.

Thus, the curvature of the piston facing surface of the segment, and the curvature of 26 the opposite surface are different and so the segment must be flexed (and thereby 27 elastically deformed) in order to be fitted to a curved cam segment support. In this 28 case, the curvature of the cam segment support facing side of the segment is greater 29 than the curvature of the cam segment support. In alternative embodiments, the segments may be intended to be retained on an inward facing cam segment support, 31 so as to provide an inward facing working surface (for example in a radial piston 32 machine where the pistons are located within the ring cam). In this case, in order that 33 the piston facing surfaces of the cam segments may be placed under tangential 34 compressive stress, the support facing surface of the segments may be provided with a greater radius of curvature than the fixing surface.

1 In comparison to known ring cams, consisting of segments secured to a support 2 structure by flanges (or other attachment means) extending to either side of the 3 working surface, attachment means (e.g. bolts) extending through the working (or 4 piston facing) surface enable the piston facing surfaces of the segments to be placed in greater compression.

7 In general, the segment 100 has been configured such that it cannot be fitted into a 8 ring cam without being elastically deformed to create compressive stress in the piston 9 facing surface, and in the working surface of the assembled ring cam. The working surface of the segment (the piston facing surface minus any parts which are typically 11 recessed below the piston facing surface of an adjacent segment in use) defines a 12 proportion, x, of a wave of a ring cam working surface. In the example shown in 13 Figure 2, this proportion is 2.00. However, the curvature of the segment underlying 14 the working surface (i.e. ignoring parts which do not engage with the rollers or other cam engaging elements in use) is a fraction, y, of 360° which is not an integral 16 multiple of x. For example, the curvature of the segment underlying the working 17 surface may be 44° and so y=0.122222. The ratio x/y = 16.3636(36) in this example, 18 which is not an integer. Thus, the working surface provided by the segment is 19 mismatched with the curvature of the segment itself and the segment cannot be used to form a ring cam having only other segments of corresponding shape, as a ring cam 21 should have an integral number of waves.

23 Segment 100 is also provided with (optional) cross apertures 114, which extend 24 through the segment generally parallel to the axis of rotation of the assembled ring cam and the piston facing surface. The cross apertures provide zones of greater 26 compressibility than the surrounding material of the segment and facilitate 27 deformation of the segment, enabling a given tangential compressive force to be 28 generated with a lower fixing force. One or more cross apertures may be positioned 29 and dimensioned to achieve a desired distribution of tangential compressive forces through the segment. For example, the position and dimensions of the cross 31 apertures may be selected so as to concentrate compressive forces on the leading 32 surface, or the steepest portions of the leading surfaces.

34 In some embodiments, the ring cam segments are formed with the cross apertures 114 located in the region of whichever of the leading face or the trailing faces of the 36 waves which make up the ring cam the pistons will exert most force during operation.

37 This force varies cyclically during active cycles as fluid flows into and out of working 1 chambers in phased relationship to cycles of working chamber volume. The variation 2 in pressure during cycles of working chamber volume depends on the function of the 3 machine. If the machine is a pump, the apertures are located in the leading faces, so 4 that the rollers pass over the apertures during contraction strokes, when the respective working chamber may be in fluid communication with the high pressure 6 manifold, and the pressure within the working chamber is above the pressure of the 7 low pressure manifold as a result. If the machine is a motor, the apertures are 8 located in the trailing faces, so that the rollers pass over the apertures during 9 expansion strokes, when the respective working chamber is receiving working fluid from the high pressure manifold. In both cases, during an active cycle, the working 11 chamber is open to the high pressure manifold whenever the working chamber 12 passes over a cross aperture.

14 In practice, it is the curvature of the cam segment support between fixing points which is important and the cam segment support may not have a continuous curvature, or 16 even be a continuous surface as shown in the figures.

18 Although the cam segments are illustrated having cooperating formations in the form 19 of a tongue at a first end and two tongues defining a groove at the other end, any of a wide range of other arrangements are possible. Tongues may be straight, curved, or 21 generally triangular, for example. The cam segments may have a single tongue at 22 either end, which tongues are adjacent each other in the assembled device, thereby 23 forming the interlocking region in use.

In some embodiments, the ring cam segments are formed with the apertures 4 26 located in only whichever of the leading face or the trailing faces of the waves which 27 make up the ring cam the pistons will exert least force during operation. This force 28 varies cyclically during active cycles as fluid flows into and out of working chambers 29 in phased relationship to cycles of working chamber volume. The variation in pressure during cycles of working chamber volume depends on the function of the 31 machine. If the machine is a pump, the apertures are located in the trailing faces, so 32 that the rollers pass over the apertures during expansion strokes, when the 33 respective working chamber is in fluid communication with the low pressure manifold, 34 and the pressure within the working chamber is at or below the pressure of the low pressure manifold as a result. If the machine is a motor, the apertures are located in 36 the leading faces, so that the rollers pass over the apertures during contraction 37 strokes, when the respective working chamber is venting working fluid to the low 1 pressure manifold. In both cases, the working chamber is sealed from the high 2 pressure manifold whenever the working chamber passes over an aperture. The 3 apertures are examples of discontinuities in the working surfaces and other 4 discontinuities, such as the interlocking regions between adjacent segments, may also be distributed in the same way.

7 With reference to Figure 3, in some embodiments, apertures 4 are provided in only 8 some of the trailing faces. The cam segment of Figure 3 is especially useful in a 9 fluid-working machine operable typically as a pump, but also operable as a motor in some conditions (for example to provide a positioning function). Thus, during 11 pumping, the rollers only pass over apertures while the respective piston cylinder is 12 expanding and the pressure within the working chamber is relatively low. However, 13 during motoring, although the pressure within the working chamber will be relatively 14 high when the rollers bear on the trailing faces during active motoring cycles, the active motoring cycles are selected to coincide with the rollers passing over the 16 trailing faces, or those portions of the trailing faces, of the waves lacking apertures.

17 Instead, the working chambers always execute idle strokes with no net displacement 18 of working fluid, during cycles when they bear on trailing faces having apertures.

19 This restricts the maximum throughput of working fluid during motoring but there are numerous applications where it is acceptable for maximum displacement during 21 motoring to be less than maximum displacement during pumping, for example, a 22 machine driven by the blades of a wind turbine will typically be operated as a pump 23 but could occasionally be driven as a motor to control the location of the blades, e.g. 24 for maintenance. In order to time active cycles to coincide with the rollers passing over trailing faces the controller refers to a database of which trailing faces include 26 apertures (and where the apertures are located), and continuous measurements of 27 the angular position of the ring cam received from shaft sensor 18, and takes this into 28 account on each selection of the volume of working fluid to be displaced by each 29 working chamber on each successive cycle of working chamber volume.

31 In some embodiments, the controller may cause a working chamber to execute a 32 partial motoring cycle in which the controller closes the high pressure valve before 33 the roller bears on an aperture (or an interlocking region, or any other discontinuity in 34 the working surface), so that the pressure within the working chamber is limited below a threshold when the roller bears on an aperture (or an interlocking region).

1 In some embodiments, the controller may allow a working chamber to execute a 2 motoring cycle while the corresponding roller passes over a trailing face including an 3 aperture only if the pressure within the high pressure manifold is below a threshold, in 4 which case the force bearing on the ring cam through the roller is anyway not excessively high.

7 Optionally, the fluid-working machine is also operable to function as a pump in a 8 second (opposite) direction of rotation. In this case the leading faces (when the fluid 9 working machine is rotating in a first direction) become the trailing faces (when the fluid-working machine is rotating in the second direction) and the trailing faces (when 11 the fluid working machine is rotating in a first direction) become the leading faces 12 (when the fluid-working machine is rotating in the second direction). In some 13 embodiments, when rotating in the second direction, the controller may allow a 14 working chamber to execute a pumping cycle while the corresponding roller passes over a trailing face including an aperture only if the pressure within the high pressure 16 manifold is below a threshold, in which case the force bearing on the ring cam 17 through the roller is anyway not excessively high. In some embodiments, when 18 rotating in the second direction, the controller may allow a working chamber to 19 execute a partial pumping cycle in which the controller closes the low pressure valve after the roller bears on an aperture (or an interlocking region), so that the pressure 21 within the working chamber is below a threshold when the roller bears on an aperture 22 (or an interlocking region).

24 For a machine operated predominantly as a motor, and in some conditions as a pump, the apertures can be located in some of the leading rather than some of the 26 trailing faces.

28 The ring cam further comprises side plates (on one or, more preferably, both sides of 29 the ring cam) extending around the circumference of the ring cam (and each side plate typically also abutting an edge of the working surface of the ring cam), which 31 prevent the rollers from sliding off the wave-like surface of the ring cam. In 32 embodiments with two or more ring cams, there may be one side plate positioned 33 intermediate two cam rings which functions to prevent rollers from sliding off both 34 cam rings. Alternatively, each of the two or more cam rings may have separate side plates.

1 The side plates may be unitary, or may be segmented as shown in Figure 3. In the 2 embodiment shown in Figure 3, ring cam segment 7 is secured to a side plate 3 segment 120 (and typically to two side place segments, to either side of the wave like 4 surface of the segment). In alternative embodiments there may be fewer, or more, side plate segments disposed around each side of the circumference of the or each 6 ring cam than there are ring cam segments.

8 The side plates are held to the segment 7 by bolts 36 extending through cross bolt 9 holes. The bolts may each extend through more than one ring cam (or ring cam segment) or more than one side plates (or side plate segments).

12 The side plate segments of the ring cam may be angularly offset from the cam 13 segments so that each side plate overlaps two (or more) segments of the assembled 14 ring cam. Thus, and in the assembled ring cam, the joint between side plate segments does not align or overlap with the joint between segments and the 16 overlapping portion 122 of a side plate segment secured to a ring cam segment may 17 be used to axially (i.e. with respect to the shaft) align the cam segment to an adjacent 18 cam segment (for example during assembly and maintenance, or to reduce wear 19 caused by motion of adjacent ring cam segments in relation to one another, when forces are applied to the wave like surface, in use). In some embodiments the side 21 plates may be fixed to the shaft, or fixed relative to the valves and working chambers 22 such that the cam segments move between the side plates.

24 The ring cam of the present invention is especially useful as a component of large fluid working machines where access may be difficult and a long ring cam working life 26 is important. For example, the ring cam may be part of a pump within the nacelle of a 27 wind turbine tower (which are typically in excess of 50 m in height), or an offshore 28 wind turbine tower, with the drive shaft coupled to the blades of a wind turbine, and 29 where it is not practicable to remove the fluid-working machine, or the wind turbine blades, for maintenance or repair.

32 Further variations and modifications may be made within the scope of the invention 33 herein disclosed.